1. Field of the Invention
The present invention relates to mounts for optical elements on vacuum sealed chambers that can be rotationally adjusted; and particularly to such mounts for gas discharge tubes used in ion lasers.
2. Description of Related Art
Ion lasers, such as the Spectra-Physics Model 2040 argon ion laser, use gas discharge tubes to provide laser gain. The gas discharge tube confines a low pressure gas in a vacuum tight chamber which includes an anode and a cathode. Electric current between the anode and cathode induces a plasma which provides laser gain. Windows are provided at either end of the gas discharge tube, allowing light to escape without breaking the vacuum tight seal.
For minimum transmission losses through the windows in polarized laser systems, Brewster windows are typically mounted on the gas discharge tube. Because discharge tubes in many embodiments generate significant amounts of ultraviolet radiation, the Brewster windows are often made of UV resistant material, such as crystalline quartz. Crystalline quartz is less susceptible to damage from ultraviolet radiation than is fused quartz, or other materials that could be used for the Brewster windows. Also, it provides good transmission at the lower end of wavelengths generated by typical ion lasers.
In mounting Brewster windows, the surfaces of the windows must be transversely aligned so that the angle of incidence of light on the window is essentially Brewster's angle for the wavelength of interest, within a reasonable tolerance of, for example, .+-.2.degree.. The Brewster windows must also be aligned rotationally, because each Brewster window will establish a plane of polarization. Thus, the Brewster window must be rotationally aligned so that plane of polarization for the window matches the desired polarization of the resonant cavity. This rotational alignment can be critical because significant loss occurs with very small misalignments. Crystalline quartz windows are particularly sensitive to misalignment because of birefringence. It is found, for instance, that a misalignment of approximately 0.5.degree. causes greater than 5% loss for a typical system, depending on the wavelength and other factors.
This rotational alignment presents a difficult manufacturing problem for prior art gas discharge tubes. In the prior art tubes, the optics are placed on rigid mounts that are welded or otherwise rigidly mounted on the gas discharge tube. Optical alignment tooling is required during these manufacturing steps in order to insure adequate rotational alignment. Typically, this involves moving the parts from one place on the manufacturing floor to an alignment test site in order to accomplish the manufacturing step of attaching Brewster windows to the gas discharge tube. Therefore, it is desirable to provide a mount for an optical element on a gas discharge tube, which allows rotational alignment after assembling the gas discharge tube.
The requirements of gas discharge tubes, such as cleanliness and tolerance of bakeout at high temperatures, further complicate the design of the optic mount. Thus, prior art rotatable mounts which use low temperature materials like elastomers cannot be used in this environment. Vacuum bellows may provide the required cleanliness and high bakeout temperature tolerance, but are expensive and impractical for commercial laser systems.
Accordingly, it is desirable to provide a rotatable, bakeable vacuum sealed mount for optical elements, which can be used on an endbell assembly for a gas discharge tube in an ion laser and that is inexpensive and easily manufacturable.